What are Piwik archive tables? (no replies)

I noticed piwik_archive_blob_YYYY_MM and piwik_archive_numeric_YYYY_MM being created each month.
Can someone tell me what they are used for?

I assume it is for some archiving but I'm not sure if the data is being moved from log_visit and log_visit_action.
I am asking b/c we have some queries that are working directly with the piwik tables.